"I just thought it was a pathetic attack," Ms Collins told The AM Show on Wednesday, telling Duncan Garner she's absolutely sure it wasn't someone from the National Party who leaked the information to O'Brien.

"You guys are great - you come along and ask us who did it when you know who did it," Ms Collins said when Garner asked if she knew who had leaked the information.

Garner later asked O'Brien if she knows who leaked the information to her, to which she said "no comment", adding that she protects her sources.

But she did says it's "extraordinary" that Simon Bridges is calling for a High Court judge to preside over a full-blown forensic investigation into the leak - information which was due to come out anyway.

"Speaking of wasting taxpayer money, that doesn't point to the behaviour of a man who wants to be Prime Minister," she said, comparing him to the likes of Winston Peters who came after those who leaked information of his superannuation details.

Ms Collins said she doesn't believe it's anyone in the National Party who leaked the spending details, but said she wants to get to the bottom of it and "find out who had that information and who gave it to Tova O'Brien."

"I think that Tova does know who leaked the information, and she's made a couple of comments to our people that have certainly made us think in a particular direction, and it's definitely not the National Party."

An investigation has been launched into the leaked information, which revealed Mr Bridges spent $113,000 on Crown limos and accommodation in the past three months, during his 'getting to know Simon' roadshow across New Zealand.

The entire National Party caucus had seen the expenses, after the Whips and Leaders Office received the information last week. Members of Parliamentary Services may have also seen the details.

Ms Collins says she wants suspicions cleared, after some had pointed to Speaker Trevor Mallard as a possible leaker. But Prime Minister Jacinda Ardern hit back on Tuesday, saying Mr Mallard is "impartial" and it's a "serious allegation".

Ms Collins said she and Trevor have "had our run-ins in the past". She once sued him for defamation. Having said that, Ms Collins added she doesn't want to have the speaker's office or ministerial service or parliamentary service "in doubt as to their integrity".

"I realise that outside of Wellington a lot of people won't think it's that important, but it is important because it goes to democracy and it goes to the fairness and integrity of the parliamentary system," she said.

"We trust the parliamentary service, ministerial service and the speaker's office with a lot of private information and I think we do need to know where this came from."

O'Brien says it's "extraordinary" that Simon Bridges is calling for a High Court judge to preside over a full-blown forensic investigation into this leak - information which was due to come out anyway.
Photo credit: The AM Show

Mr Bridges spent $84,000 travelling the length of the country in a Crown limousine during his roadshow. He spent $919 a day - more than people earning the minimum wage take home in a week.

"No leader of the opposition in a decade has spent anywhere near this much money," O'Brien told The AM Show. Former Opposition leader Andrew Little spent $35,000 less than Mr Bridges over the same time last year.

O'Brien said it's great that Simon Bridges travelled the country getting to know New Zealanders, but that there "are ways of keeping costs down".

But Ms Collins said leading the opposition is the "hardest job in the world", and that Mr Bridges was just trying to do the right thing by getting out and meeting New Zealanders.